Download M5L5 Phylum Annelida Segmented Worms

Marine Biology
Phylum Annelida: Segmented Worms
Phylum Annelida: Segmented Worms
Phylum Annelida includes earthworms and many marine worms. Their body plan is
segmented. You might recognize these segments in
the rings you see on an earthworm. Annelids are the
first organism to have a true body cavity or a coelom,
that develops from a middle layer of tissue called the
mesoderm. The coelom of the annelids is filled with
fluid and divided by partitions. The segments act as a
hydrostatic skeleton and can be contracted in a
sequence by longitudinal muscles which lengthen and
shorten segments while circular muscles increase or
decrease the diameter of the segments. These
movements allow the segmented worms to be excellent crawlers and burrowers.
Most marine annelids are in the class Polychaeta (polychaetes). Each of the body
segments in a polychaete has a pair of flattened extensions (paddles) called parapodia
(podia means feet!). These parapodia have sharp bristles called setae. These help
with movement as well. Some polychaetes are tube worms. These tubes will be
burrowed in the mud and the worms have reduced parapodia. Some have tentacles
which allow them to filter feed of particles in the water.
In annelids, blood is moved throughout the body entirely within blood vessels.
Therefore, they have a closed circulatory system. Like the organisms listed before,
small annelids exchange gases directly through the body wall. However, larger annelids
have gills that may be located on the parapodia. These gills have capillaries that allow
for greater respiratory exchange of gases.
Other classes of marine annelids include the Oligochaetes, which are the marine
version of the earthworm and Hirudinea, or leeches. Leeches are more typically
freshwater organisms, but marine species can be found attached to fish and
invertebrates. They are highly specialized annelids that are distinguished by their
suckers (for attachment) and the lack of parapodia.
Click the link to see a video of a "Feather Duster" worm (polychaete) at
http://www.youtube.com/watch?v=kIoN2PCvL4o. It is a tube worm and the body of the
worm is in the tube, most of which is buried in the sediment. The tentacles come out to
filter food from the water and take it to the worm's mouth.
Photo Attribution
Description: Earthworm; Wikipedia.org
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